This invention relates to a method for eliminationg ammonia nitrogen from sewage by contacting sewage with silica gels.
There are large amounts of BOD (Biochemical Oxygen Demand), COD (Chemical Oxygen Demand), ABS (Alkyl Benzyl Sulfonates), organic nitrogen and ammonia nitrogen in the waste drained from houses or factories.
The treatments of sewage can be divided into the primary, the secondary and the tertiary treatments.
The primary treatment is sedimentation. Only a fraction of organic and inorganic suspended matter in sewage is removed by sedimentation.
The secondary treatment is an activated sludge process. Most of BOD and COD in sewage is removed by activated sludge. At the same time organic nitrogen decreases slightly.
In many cases one of the tertiary treatments is an activated carbon process, by which the values of COD, ABS and organic nitrogen decrease further.
The water obtained by the tertiary treatment contains less than 1 ppm of BOD, 1.about.25 ppm of COD, 1.about.6 mg/l of total organic carbon, 0.01.about.0.5 mg/l of ABS, 0.1.about.1 mg/l of PO.sub.4, and 1.about.2 mg/l of organic nitrogen. It is very clean.
However there is a contaminant which is not eliminated by the abovementioned treatments. It is ammonia nitrogen. It is very difficult to eliminate ammonia nitrogen from sewage because it is quite soluble in water and very stable as a nitrogen compound.
Ammonia nitrogen overnourishes rivers, lakes and seas, which causes red tide phenomena and facilitates abnormal generation of seaweed. Fish and shells suffer vast damages from the excess ammonia nitrogen.
Furthermore the water containing ammonia nitrogen is not good as a source of service water. It causes corrosion of metals.
This invention will solve the difficulties by eliminating ammonia nitrogen from sewage and making clean water. This invention gives a new method for eliminating ammonia nitrogen which is best for the tertiary treatment of sewage.
Throughout this specification "ammonia nitrogen" is defined as the nitrogen atom N in the ammonia molecule NH.sub.3. Nitrogen is included in various forms in materials, for example as protein in animals or plants, nitrites, nitrates, nitric acid, ammonia or atmospheric nitrogen. But the nitrogen contained in sewage mostly exists as ammonium ions. Therefore the elimination of ammonia nitrogen is not easy. The applied water of the invention is the water obtained after secondary treatment.
The water obtained after the secondary treatment contains less than 1 ppm of BOD, 20.about.60 ppm of COD, 8.about.18 mg/l of organic carbons, 0.4.about.2.9 mg/l of ABS, 2.about.4 mg/l of organic nitrogen and no suspended matter. However the concentration of ammonia nitrogen is unaltered and comes to 20.about.30 ppm.
Some methods for eliminating ammonia nitrogen have already been found and used.
For example,
(1) Ammonia Stripping
(2) Ion Exchange
(3) Biological Nitrification-Denitrification
(4) Breakpoint Chlorination
The ammonia stripping process includes the high pH conditioning nearly to 11 and the stripping with large volumes of air 3000 times as much as the volume of the sewage. In the process ammonia is removed by the air and discharged as ammonia gas to the atmosphere.
A disadvantage of the process is that it requires a large amount of air. Thus the process needs large cooling towers and causes high operating costs. Moreover it is necessary to treat a large amount of ammonia gas. But the treatment of ammonia gas is very difficult because it has been diluted by a large amount of air. Sometimes it may cause secondary air pollution.
An ion exchange process selectively eliminates ammonium ions from sewage by passing the ions directly through a zeolite bed. This process has a difficulty with the regeneration of zeolite. Zeolite is regenerated with a solution of caustic soda or caustic potash added with Nacl at near pH 12. The spent regenerant containing high concentration of ammonium ion (NH.sub.4.sup.+) is air-stripped to eliminate ammonia.
In the case of thermal regeneration of zeolite, Ammonia is discharged to the atmosphere as a gas by heating the zeolite.
However both methods of the regeneration of zeolite are incomplete.
The alkaline-Nacl regeneration cannot entirely eliminate ammonium ions from the spent regenerant. In the case of the thermal regeneration hydrogen ions H.sup.+ remain in the regenerated zeolite which decreases the efficiency of ammonia elimination. An additional disadvantage is that the zeolite is damaged by heating.
Biological Nitrification-Denitrification is a process to oxidize ammonia nitrogen to nitrate nitrogen by nitrification bacteria under aerobic conditions and to reduce nitrate-nitrogen to nitrogen gas N.sub.2 under anaerobic conditions. The N.sub.2 is then discharged to the atmosphere.
This method is ideal. It causes no secondary pollution, because ammonia is converted to nitrogen gas.
However as this method utilizes bacteria, it needs big installations where it is difficult to maintain optimum operating conditions Especially in winter, Ammonia removal efficiency decreases significantly because of low temperature. Furthermore, when nitrate nitrogen is reduced to nitrogen gas, nourishment for bacteria, for example, methanol must be added. Thus the operating cost is high.
In breakpoint chlorination chlorine gas is blown into sewage. Chlorine gas becomes a hypochlorous acid ion in the water; and, the ammonium ion is finally oxidized to nitrogen gas via chloramines.
The advantage of the required method is that the equipment is comparatively simple.
But the method needs a large amount of chloride--about eight times as much as ammonia nitrogen in weight. Furthermore chlorine is fatally poisonous and chloramines (intermediate materials; NH.sub.2 Cl, NHCl.sub.2, NCl.sub.3) are also poisonous. These poisonous materials must be completely eliminated.
Activated carbon is used in water treatment wherein ammonia in gas-phase is sightly adsorbed by activated carbon. However as before-mentioned, the ammonia adsorption ability of activated carbon is low. This method cannot bring about a satisfactory result.
Totally differing from these methods, this invention uses silica gel to adsorb ammonia nitrogen.
The main component of silica gel is written as SiO.sub.2. Silica gel is a glassy porous material which has neither solubility in water, corrodability nor deliquescence.
Silica gel has many small holes which are formed by networks of colloidal particles of silica which have enormous effective surface areas. In A-type silica gel (standard type), the surface area is 700.about.800 m.sup.2 /g; the diameter of the small holes is 22.about.26 A; and, the volume of the small holes is 0.4 cc/g. In B-type silica gel (low concentration type) the surface area is 300.about.500 m.sup.2 /g; the diameter of the small holes is 70.about.150 A; and, the volume of the small holes is 0.8 cc/g.
On account of the enormous number of small holes, silica gel can adsorb water very well by capillary action. Indeed silica gel is a most frequently used hygroscopic material. Silica gel has various uses as an adsorbent of moisture--a dehumidifier or dryer. It is indispensable for preservation of foods; dehumidifying of storehouses; or, the prevention of mold, putrefacation or corrosion of goods in transportation. Silica gel is used for dehydrating and refining hydrogen gas, oxygen gas, chlorine gas, carbonic acid gas or other industrial gases.
Indeed silica gel is a most desirable dryer, as it is tasteless, scentless, harmless to human beings and chemically stable. Moreover it is desirable in that it handles very easily and can be easily regenerated by heating.
Both A-type silica gel and B-type silica gel adsorb moisture very well, but their constitution differs a little. Because A type silica gel has greater surface area, it has a great ability to adsorb moisture by the affinity of the hydroxide groups on the surface. It is suitable at low humidity.
B-type silica gel has a larger volume of small holes. Thus it is suitable for dehumidifying at high humidity.
It is well known that silica gel adsorbs moisture by capillary action, but nobody has recognized that silica gel can adsorb ammonia. The inventor on the other hand has discovered that silica gel selectively adsorbs ammonia and, that discovery has led to the invention about to be further described.